Self-progragatng high-temperature synthesis of lanthanide orthochromites LnCrO_3

摘要

Lanthanide orthochromites of general formula LnCrO_3 (Ln chemical bounds La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Lu) have been made by Self-propagating High-temperature Synthesis (SHS). Four different series of SHS samples were produced; series 1-SHS of Ln_2O_3, Cr and CrO_3 with NaClO_4 of Ba(ClO_4)_2 in air; series 2-SHS of Ln_2O_3, Cr and CrO_3 in oxygen; series 3 -SHS of NlCl_3, CrCl_3 and NaO_2 or Na_2O_2 in air; series 4-SHS of Ln(NO_3)_3, Cr and melamine in air. Maximum reaction temp[erature T_c and propagation velocity U_c were measured for all reactions. Selected series 1 reactiolns were studied by DTA these showed an exotherm at 450 deg C. X-ray powder diffraction showed, that in all series of SHS reactions a single phase orthorhombic perovskite structure was produced. Systematic changes in the lattice parameters, cell volume and pycnometrical densities wer seen as a function of lanthanide atomic number. SHS prepared LnCrO_3 samples were all shown to be weak antiferromagnets. IR reflection spectra for the LnCrO_3 materials gave four broad absorbtions in the region 800-200 cm~(-1).